It's your chin. Tiger says you should keep your chin up all the way through your swing.
I'll let you read the article for the details -- it's relatively short -- but here are the four bad effects Tiger says burying your chin in your chest at setup can cause:
- It restricts your shoulder turn during the backswing.
- It inhibits your weight shift during the downswing.
- It affects your spine angle during the downswing.
- It messes up your balance all the way through your swing.